Concrete Flexural Strength (Modulus of Rupture): Beam Test Guide
Published November 2025 · 6 min read · Elipslab Technical Team
What Is Flexural Strength?
Flexural strength (also called modulus of rupture, MR) is the maximum stress a concrete beam can resist before breaking under bending. Unlike compressive strength, which is measured on cylinders or cubes, flexural strength is measured on beam specimens — reflecting how concrete performs in pavements and slabs subjected to bending loads.
Concrete has high compressive strength but low tensile/flexural strength (typically 10–15% of fc'). Road pavement design uses MR as the primary design parameter rather than compressive strength.
Test Methods
- ASTM C78 — Third-point loading: Load applied at the third points of the span. If fracture occurs within the middle third, the formula MR = PL/bd² applies directly. This method produces a uniform moment zone and is preferred for research and specification compliance.
- ASTM C293 — Center-point loading: Single load at midspan. Simpler setup but produces stress concentration at the load point. MR = 3PL/2bd².
- EN 12390-5: Three-point (center) loading on 150×150×600 mm beams. Uses the same formula as C293.
Specimen Dimensions
| Standard | Beam Width × Depth × Length | Loading |
|---|---|---|
| ASTM C78 | 150×150×530 mm (min) | Third-point |
| ASTM C293 | 150×150×530 mm (min) | Center-point |
| EN 12390-5 | 150×150×600 mm | Center-point |
Equipment
- Flexural Testing Frame: Universal testing machine or dedicated concrete beam tester with the appropriate load span. For 150 mm beams, machine capacity of 50–100 kN is usually sufficient.
- Loading Jig: Steel bearing blocks and rollers, hardened and ground. Third-point loading requires four contact points (2 load, 2 support); center-point uses three.
- Beam Molds: 150×150×600 mm steel molds for casting EN 12390-5 specimens.
Typical Values
Normal concrete typically achieves MR values of 3.5–5.0 MPa at 28 days. Rigid pavement design commonly specifies a minimum MR of 4.5 MPa. High-performance concrete can reach 6–8 MPa.
Concrete Flexural Testing from Elipslab
Elipslab supplies concrete beam testing frames and complete third-point loading jig sets compliant with ASTM C78 and EN 12390-5. Compatible with standard 50–200 kN universal testing machines. Manufactured in Ankara, Turkey.